MMP9 and infection: To further elaborate on the gene expression profile and tissue damage at the protein level, we determined the serum levels of MMP-9 and MMP-3 at several days p.i. At 5 d.p.i. the serum levels of MMP-9 in the VACV-WR infected mice were significantly higher than on earlier days or following infection with the vaccine strains (Figure 7C, VACV-Lister).